Militants tied to the Somali terror group al-Shabab made the threat against Christians last month.

The group Muslim Renewal is suspected in the Feb. 17 assassination of Father Evarist Mushi. He was gunned down outside Saint Teresa's Catholic Church in Zanzibar.

Al-Shabab-linked militants also may have been responsible for the killing of Somali Pastor Abdi Welli last month in the Kenyan town of Garissa.

"Many more will die. We will burn homes and churches. We are not finished...prepare for disaster," Muslim Renewal said in a statement following those killings.

Dozens of churches have been attacked in Tanzania and neighboring Kenya, including Zanzibar's Pool of Siloam Evangelical Church in Kianga. Militant Muslims burned the church Feb. 19 -- just two days after the murder of Mushi.

"They took all the plastic chairs that were here and placed them by the altar. They poured gasoline over the chairs and lit them afire," Siloam Pastor Elijah Israel told CBN News.

"And why do you feel they are trying to destroy the church?" CBN News asked him.

"They've said it in public, it's no longer hidden. They don't want the Christians..." he replied.

It wasn't the first time the church suffered destruction. A mob wielding sledge hammers knocked down the church walls in November 2011.

Still, Pastor Israel said Muslim extremists will not succeed in driving the church away.

"If the building is no longer here, we Christians will still worship God on this soil. We will remain until the glory of God is manifested here," he vowed.

In neighboring Kenya, Bishop Jeremiah Kibobi, with Visions of Glory Church, said his church members are on alert. Armed police will guard the church building near Nairobi this Easter Sunday.

"The church has been frightened because of what has been happening lately, the past two years in Kenya," Bishop Kibobi said.

Nevertheless, he added, "The Word of God says no weapon formed against you shall prevail, and the Word of God also says if my people who are called by my name will humble themselves and pray and repent of their sins."

"God is going to restore our land when the church is united in prayer," the bishop said. "The church will overcome."